Methods of fabricating thin film structures by imaging through the substrate in different directions

ABSTRACT

A method of fabricating thin film structures on the surface of a transparent substrate (10) in which a light shielding pattern (20, 54) is provided adjacent the opposing substrate surface and in which deposited thin film layers are photolithographically patterned by directing radiation onto the light shielding pattern and by varying the angle of the exposing radiation for respective layers whereby the thin film layers are patterned differently while using the same light shielding pattern. Various thin film structures can be fabricated inexpensively and reliably using this approach together with standard processing techniques such as the use of temporary layers and lift-off procedures. In particular, by appropriate design of the light shielding pattern and selection of deposited materials, active matrix arrays, e.g. comprising MIMs or TFTs with associated conductors, for use in liquid crystal display devices can be produced. The light shielding pattern can be formed directly on the substrate enabling pliable substrates to be used.

BACKGROUND OF THE INVENTION

This invention relates to a method of fabricating on one surface of atransparent substrate a thin film structure comprising a plurality ofthin film layers in predetermined patterns, which method comprises thesteps of providing a light shielding pattern adjacent the opposingsurface of the substrate and photolithographically patterning a firstthin film layer deposited over the one surface according to the lightshielding pattern using radiation directed onto the light shieldingpattern.

The invention relates also to thin film structures fabricated therebyand display devices incorporating such structures.

Thin film structures fabricated on transparent substrates are used in avariety of products such as display devices, solar cells, light sensingdevices, memory devices, and printing devices. The invention isconcerned with fabricating thin film structures suitable for suchproducts and particularly, although not exclusively, with fabricatingarrays of thin film switching elements for use in active matrix displaydevices, together with associated address conductors and picture elementelectrodes. Typically, the thin film switching elements comprise two orthree terminal devices such as MIM devices and TFTs respectively whichare connected between address conductors and picture element electrodeson a common transparent substrate.

A method of the kind described in the opening paragraph for fabricatingthin film MIM devices on a transparent substrate for use in an activematrix liquid crystal display device is disclosed in U.S. Pat. No.4,683,183. In this method, metal strips constituting a set of addressconductors are defined on the substrate using a mask. Thereaftersuccessive layers of insulating and conducting materials are depositedover the substrate surface and these layers are patterned by means of aphotolithographic process which involves depositing a photoresist layerover the layers to be defined, applying a photomask to the rear surfaceof the substrate and illuminating the substrate from behind whereby thepattern of the superimposed insulating and conducting layers obtained isdetermined in part using a self alignment technique in which the metalstrips serve as a mask to define a MIM structure and also in part by thephotomask to define the length of the MIM structure and the area of thepicture element electrode. Thus, the fabrication of the MIM device andits associated picture element electrodes entails using a mask to definethe metal strips, those strips then themselves being used as a mask, andalso a photomask.

In the English language abstract of Japanese Patent Application No.62-132367 there is disclosed a method of fabricating TFTs on atransparent substrate for use in a liquid crystal display device whichinvolves disposing a light shielding film layer on the rear surface ofthe substrate and photolithographically patterning a layer deposited onthe upper surface by illuminating the substrate from behind with thelight shielding film serving as a mask defining regions of the depositedlayer which constitute source and drain terminals of the TFT. Furtherlayers are then deposited which would require additional patterningprocesses using further masks to complete the TFT.

In requiring a plurality of patterning processes involving separatemasks, these known methods suffer from disadvantages similar to thoseexperienced with more common methods of fabricating of TFTs or MIMs fordisplay devices or the like entailing a series of conventionalphotolithographic processes needing a set of several photo masks todefine patterns in layers of different materials. To increase thedisplay area of a display device, for example, it is necessary toproduce correspondingly sized arrays of TFTs or MIMs. However, as thesize of the array is enlarged it becomes increasingly more difficult andmore expensive to provide substrates that remain sufficiently rigidthrough the thermal cycles in the fabrication processes to ensureaccurate alignment between successive photomasks and the substrate. Asimilar problem exists when fabricating smaller sized but higher densityarrays. Of course, such known methods are not well suited to fabricatingthin film structures on flexible substrates.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method offabricating thin film structures on a transparent substrate which can beemployed to alleviate the aforementioned problem at least to someextent.

It is another object of the present invention to provide a method offabricating thin film structures, such as switching element arrays, on atransparent substrate which requires a minimal number of masks.

It is a further object of the present invention to provide thin filmstructures such as switching element arrays together with addressconductors and electrodes on a transparent substrate suitable for use inactive matrix display devices in a convenient and inexpensive manner.

According to one aspect of the present invention a method of the kinddescribed in the opening paragraph is characterized by the step ofpatterning photolithographically a second thin film layer deposited overthe one surface of the substrate using radiation directed onto the lightshielding pattern in a direction with respect to the substrate differentto that used for patterning said first thin film layer.

The invention involves the recognition that different layer patterns canbe created by using one light shielding pattern, serving as a mask, andchanging the direction of illumination when defining the differentlayers. It will be appreciated that as a result of using differentdirections between the exposure beam and the substrate the patternobtained in the two thin film layers, while both being dependent on thelight shielding pattern, will be different to one another. Consequently,two thin film layers can be defined into respective patterns using thesame light shielding pattern, rather than using two separate masks. Thedifference in patterns obtained in a simple case can be in the form ofan offset between the defined regions of the two layers, the extent ofoffset being dependent on the difference in illumination directions andthe distance between the light shielding pattern and the resist layerused in the photolithographic patterning process. In the case of thesecond layer being deposited and patterned after patterning of the onelayer then, depending on the material used for the one layer and itstransparency to the radiation, the patterned one layer may also play apart in the patterning of the second layer.

This method can be used to considerable advantage in simplifying thefabrication of thin film structures.

A third thin film layer may be deposited over the one surface of thesubstrate and patterned photolithographically using radiation directedonto the light shielding pattern in a direction different to that usedin the photolithographical patterning of said first, and/or second thinfilm layers. The pattern of the third layer will then be different to atleast one of the other layers, and may in addition also be dependent onthe pattern of the underlying layer(s).

A further one or more thin film layers may be deposited successivelywith one of said first, second or third layers and photolithographicallypatterned simultaneously with that layer. In this way, two or morelayers of different materials are patterned substantially identicallyand a pattern of the combined, and superimposed, layers is formed on thesubstrate.

One or more of the deposited thin film layers may be patterned usingradiation directed onto the light shielding pattern from more than onedirection. This enables further differences in the patterning of anindividual layer or layers to be obtained. The one or more layerspreferably are patterned using radiation in two substantially opposingdirections which lie in a plane substantially perpendicularly of the onesurface of the substrate. By suitably choosing these substantiallyopposing directions with respect to radiation opaque parts of the lightshielding pattern which have comparatively small dimensions in relationto those directions, for example narrow strip regions, the definition ofparts in the thus patterned layer corresponding therewith can beselectively excluded.

As a further enhancement in fabricating thin film structures, atemporary thin film layer pattern may be provided over the substratesurface by depositing a thin film layer and patterning the layerphotolithographically using radiation directed onto the light shieldingpattern, which temporary layer pattern is subsequently removed followingthe deposition of one of said thin film layers, for example by selectingetching, so as to lift off immediately overlying regions of that onethin film layer. In this way still further different layer patterns canbe produced.

The term light shielding pattern used herein is intended to mean apattern Which is opaque to the exposing radiation which preferably isU-V light, although visible light and possibly x-rays may be used, andthe term should be construed accordingly.

The above techniques conveniently enable two or more thin film layers tobe patterned separately using just one light shielding pattern andconsequently they avoid the kind of problems experienced in knownprocesses which use a plurality of separate masks, and in particularthose problems associated with the need for accurately aligning eachmask. The techniques offer the advantage that by appropriate selectiveutilisation of one or more of them various thin film structures can befabricated in a simple manner reliably and inexpensively by reducing thenumber of masks entailed compared with known processes for producingsimilar structures. In particular and desirable embodiments of themethod of the present invention, then by suitably selecting the lightshielding pattern, the materials of the deposited layers, and theradiation exposure directions used in photolithographically patterningof the deposited layers, an array of MIMs or TFTs together withassociated address conductors and pad electrodes can be fabricated on atransparent substrate suitable for use for example in active matrixliquid crystal display devices or other products such as memory devices.The method requires in effect just one mask, constituted by the lightshielding pattern, compared with the multiplicity of masks normallydemanded. Problems with registration and the need for accurate alignmentnecessary in known multiple mask, multiple exposure fabricationprocesses used for such structures are thus avoided. Comparativelylarge, or high density, arrays can be achieved without the need to usevery sophisticated photolithographic equipment.

The light shielding pattern may be arranged adjacent, but spaced from,the opposing surface of the substrate. Preferably, however, the lightshielding pattern is carried on the opposing surface. Preferably, thelight shielding pattern comprises a thin film pattern of radiationopaque material, for example, metal, formed on the opposing surfacewhich pattern can readily be obtained by photolithographically defininga deposited layer using a master mask. Relative movement between thelight shielding pattern and substrate is then prevented. As the lightshielding pattern and the substrate can move together as one the needfor high substrate rigidity is removed and greater freedom is allowed inthe choice of substrate material. Because the light shielding pattern isfixed to the substrate, and can bend and stretch with the substrate,problems due to the effects of thermal cycling in the fabricationoperations, for example distortions in glass substrates, are avoided.Importantly, pliable substrates, such as flexible foils, can be used aswell as rigid glass substrates.

According to another aspect of the present invention, there is provideda thin film structure fabricated in accordance with the one aspect ofthe invention.

According to a further aspect of the present invention there is provideda liquid crystal display device comprising a thin film structure on atransparent substrate, including for example an array of two terminalnon-linear devices or TFTs, fabricated in accordance with the one aspectof the invention.

BRIEF DESCRIPTION OF THE DRAWING

Methods of fabricating thin film structures on transparent substrates,and particularly arrays of MIMs and TFTs suitable for use in activematrix display devices, and thin film structures and display devicesproduced thereby, in accordance with the present invention will now bedescribed, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1a, 1b, 2a and 2b, and 3a and 3b schematically illustraterespectively three different patterning techniques employed inembodiments of the invention;

FIG. 4 is a plan view of part of the rear surface of a substrate showinga light shielding pattern formed thereon for use in a first method offabricating thin film substrates according to the invention;

FIGS. 5a to 5e are plan views of part of the front surface of thesubstrate showing layer patterns at various stages in this first method;

FIG. 6 is a plan view of part of the front surface of the the substrateshowing a completed thin film structure fabricated by the first method;

FIG. 7 is an enlarged view of part of the structure shown in FIG. 6;

FIGS. 8 and 9 are cross-sectional views along the lines A--A and B--Brespectively in FIG. 7;

FIG. 10 illustrates the circuit configuration of a display deviceincorporating the thin film structure produced by this first method;

FIG. 11 is a plan view of part of the rear surface of a substrateshowing a light shielding pattern formed thereon for use in a secondmethod of fabricating thin film structures according to the invention;

FIGS. 12a to 12c are plan views of a part of the front surface of thesubstrate showing layer patterns at various stages in the second methodof fabricating thin film structures;

FIG. 13 is an enlarged plan view showing a part of the completed thinfilm structure produced by the second method; FIG. 14 is a sectionalview along the line C--C of FIG. 13; and

FIG. 15 illustrates the circuit configuration of a typical part of thethin film structure produced by the second method.

It should be understood that the Figures are merely schematic and arenot drawn to scale. In particular certain dimensions such as thethickness of layers or regions may have been exaggerated whilst otherdimensions may have been reduced. It should also be understood that thesame reference numerals are used throughout the Figures to indicate thesame or similar parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 3 illustrate in simple manner three basic layer patterningtechniques for producing respective, different patterns in a depositedlayer, and ultimately desired thin film structures, in accordance withthe invention. The patterning obtained in each technique is determinedby an identical light shielding pattern while the layer pattern producedthereby is different in each case. These individual techniques can beemployed selectively to produce layer patterns required in thefabrication of thin film structures as will be described subsequently.With regard to FIGS. 1 to 3 there is shown schematically incross-section a portion of a suitably transparent substrate, 10, forexample of glass or plastics, having opposed, parallel, planar surfaces.On the lower surface a pattern of metal or other radiation opaquematerial comprising comparatively narrow and wide regions 12 and 13 isprovided using a conventional photolithographic process in which thematerial is deposited as a continuous layer over the lower surface andis then defined, by means of exposure through a mask, to leave thepattern regions 12 and 13. This metal pattern constitutes a lightshielding pattern which is then used in the photolithographic patterningof layers deposited on the other, upper, surface of the substrate usingradiation directed toward the substrate from the side of the substrateremote from the deposited layers.

In all photolithographic processes described hereinafter standardphotoresist materials and etching agents and UV exposing radiation areutilized.

Referring to Figures 1a and 1b, a layer 11 of material to be patternedis deposited over the upper surface of the substrate 10. This layer 11is then patterned by a photolithographic process which involvesdepositing thereover a layer of positive photoresist 14, and directingsubstantially parallel light, constituting an exposure beamperpendicularly towards the lower surface of the substrate 10, as shownby the arrows A, so as to expose regions of the photoresist layer 14according to the light shielding pattern on the lower surface, removingexposed regions of the photoresist, etching the underlying regions ofthe layer 11, and thereafter removing the remaining unexposed regions ofthe photoresist to leave the structure as shown in FIG. 1b. Thisstructure comprises regions 15 and 16 corresponding respectively to theregions 12 and 13. The layer pattern 15, 16 thus obtained correspondsdirectly to, and is registered with, the light shielding pattern.

Referring to FIGS. 2a and 2b, a layer 11 is again deposited on the uppersurface of the substrate and patterned using a similar photolithographicprocess. However, in this case, the lower surface of the substrate 10 isilluminated with substantially parallel light, A, which is inclined withrespect to that surface. Consequently, the layer pattern obtained on theupper surface, FIG. 2b, comprises regions 17 and 18 which substantiallycorrespond to the regions 12 and 13 in terms of their size and relativedisposition, but is laterally offset with respect to the light shieldingpattern on the lower surface.

The technique illustrated in FIG. 3a and 3b is an extension of the abovedescribed technique. The layer 11 is again photolithographically definedbut in this case the photoresist layer is subjected to two exposuresinvolving substantially parallel light directed onto the lower surfacein different directions. In the illustrated example the radiation inboth exposures is slanted with respect to the substrate surface, asindicated by the arrows A and B but from opposing directions. Because ofthe angles of illumination at which the two exposures are performed, thelayer pattern obtained differs significantly from those of the previoustwo techniques. In particular, with the two directions of the exposingradiation being opposed, and with individual rays in the two exposureslying and intersecting one another in a plane substantiallyperpendicular to the substrate surface, the dimensions of the definedregions of the deposited layer in this plane are less than thecorresponding dimensions of the regions of the light shielding pattern.If a region of the light shielding pattern is of sufficiently smalldimension in the relevant direction the region of resist overlying thatregion will be completely exposed and consequently no correspondingregion in the layer pattern is produced. Thus, as shown in FIG. 3b, thelayer 11 is patterned to produce a region 19 corresponding to the region13 but with a reduced dimension in the plane of the section shown,whereas, as a result of the directions of the illumination in the twoexposures with respect to the region 12, the layer pattern is devoid ofany region corresponding to the region 12. In an extension of thisparticular technique, the patterning process may involve illuminationfrom more than two directions.

It is seen, therefore, that with these basic techniques the same lightshielding pattern can be made to image different patterns, andconsequently that different layer patterns can be created as a result ofchanging the direction and angle of incidence of the illumination usedin the exposure operation of the photolithographic processes. Further,while the above-described examples relate to a single layer depositeddirectly on the substrate, it will be appreciated that they can beapplied to multiple thin film layer structures. It should also beunderstood that if successive layers are deposited over the uppersurface of the substrate, these layers can be patterned, eitherindividually or in combination, with the patterning of one or more upperlayers being dependent also on the optical properties, i.e. thetransmissivity, of one or more underlying layers. When successivepatterning processes are performed using the same light shieldingpattern each time, alignment between the resulting layer patterns isensured.

The photolithographic definition of differing layer patterns on thesurface of the substrate using a single light shielding pattern on theopposing surface and by varying the angle of illumination according tothe above-described basic techniques can be used in conjunction withconventional processing and patterning techniques such as lift off andthe employment of positive and negative type photoresists to fabricatevarious thin film structures.

Examples of the fabrication of thin film structures comprising padelectrodes connected to address conductors via switching elements andsuitable for use as the active substrate component in an active matrixliquid crystal display device will now be described. FIGS. 4 to 9illustrate various stages in one embodiment of a method of fabricatingsuch a thin film structure according to the invention. This structureconsists of an array of electrodes each of which is connected to anassociated address conductor via a MIM device and constitutes the activematrix substrate of the liquid crystal display device. MIM addressedliquid crystal display devices are well known. Briefly, they comprise arow and column array of picture elements each formed by a pair ofopposed electrodes carried on respective substrates with an interveninglayer of liquid crystal material. One substrate carries a row and columnarray of individual, generally rectangular, pad electrodes constitutingthe picture elements' first electrodes and a first set of paralleladdress conductors extending therebetween. The pad electrodes of eachrow are connected to a respective address conductor via associated MIMdevices. The other substrate carries a second set of parallel addressconductors which cross the first set of address conductors and portionsthereof which overlie the pad electrodes constitute the secondelectrodes of the picture elements.

In the fabrication of the active matrix substrate, a light shieldingpattern is first formed on the lower surface of the glass substrate 10on which the thin film structure is to be built by depositing a metallayer and defining this layer photolithographically in conventionalfashion using a mask so as to leave a metal pattern. The metal pattern20, a representative part which is shown in plan view of FIG. 4,comprises an array of rectangular areas 21, a set of lines 22 extendingbetween adjacent rows of areas 21, and bridging portions 23 connectingeach area 21 to a line 22. The parts 21, 22 and 23 serve to determinethe required thin film structure as will become apparent and theirdimensions are chosen accordingly. Each pad electrode, MIM device andassociated portion of address conductor of the required structure isdefined by a respective cell of the light shielding pattern, one suchcell being indicated by the dashed lines in FIG. 4. For convenience, thefabrication of a part of the thin film structure determined by just onesuch cell will now be considered. FIGS. 5a to 5e illustrate plan viewsof the upper surface of the substrate 10 at various stages in thefabrication of the thin film structure. For the sake of clarity, thesefigures illustrate the formation of an individual layer pattern ofinterest at a particular stage and, with regard to the later stages,layer patterns defined earlier have been deliberately omitted. FIG. 6shows in plan view the completed thin film structure.

In order to simplify the description of the illumination directions usedin photolithographic processes at various stages, the directions of thecompass will be used to indicate the direction of slanted radiationexposures with the top, bottom, left and right sides of the drawingsheet corresponding respectively to North (N), South (S), East (E) andWest (W).

A layer of positive photoresist is deposited over the upper surface ofthe substrate 10 and subjected to a double exposure from the rear of thesubstrate 10 onto the light shielding pattern 20 using the technique ofFIGS. 3a and 3b with N and S slanted radiation exposures ofsubstantially parallel light. The resist layer is then developed withremoval of exposed parts, to leave the layer pattern 25 shown byhorizontal hatching in FIG. 5a. The light shielding pattern isillustrated in this Figure, and similarly in FIGS. 5b to 5e, in dashedoutline for reference. As a result of this double exposure the dimensionof the defined photoresist pattern 25 in a direction parallel with theN-S axis is less than that of the light shielding pattern 20 while thedimension in a direction parallel with the E-W axis is substantially thesame. The difference in dimensions in the N-S direction is dependent onboth the distance between the light shielding pattern and the layer ofresist and the angle of slant of the exposing radiation. Theseparameters are appropriately selected such that, apart from at theregion of the bridge portion 23, the resist material overlying the line22 of the light shielding pattern is completely illuminated, and thusremoved.

A layer of negative photoresist material is then deposited. This isexposed using substantially parallel radiation directed perpendicularlyonto the rear side of the substrate 10 and then developed, according tothe technique of Figures 1a and 1b. A metal layer, for example ofaluminium or chromium, is evaporated over the upper surface of thesubstrate, following which the remaining portions of the positive andnegative photoresist layers are removed, for example using acetone orfumic acid, with lift-off of the corresponding regions of the metallayer to leave regions 27, 28 and 29 of metal, as indicated by theslanted hatching in FIG. 5b, carried directly on the upper surface ofthe substrate 10 and determined by the light shielding pattern 20. Thewidth of the strip regions 28 and 29 thus obtained is therefore dictatedby the aforementioned illumination parameters and can be controlledaccordingly.

A layer of transparent conductive material such as ITO and a layer ofpositive photoresist material are then deposited in successioncompletely over the substrate. The resulting structure is then subjectedto a double exposure from the rear of the substrate 10 usingsubstantially parallel radiation which is slanted in E and W directionsrespectively, again using the technique of FIGS. 3a and 3b. Afterdeveloping the resist, the areas of the ITO layer which are not coveredby the remaining resist material are etched away. Thereafter theremaining regions of the resist are removed to leave regions 30 and 31of ITO material, as depicted by the slanting hatching in FIG. 5c,generally corresponding with the regions 22 and 21 of the lightshielding pattern 20, it being understood that underlying regions 27, 28and 29 of the metal layer have been omitted for clarity. As aconsequence of the double exposure used to define the ITO layer, thedimension of the region 31 in the E-W direction is less than thecorresponding dimension of the region 21 of the light shielding pattern20, while the dimension of the regions 30 and 31 in the directionparallel to the N-S axis are substantially the same as those of theregions 21 and 22. The parameters selected for the E and W exposures aresuch that the region of the resist overlying the bridging portion 23 iscompletely exposed whereby the subsequently defined regions 30 and 31 ofITO are physically separate.

In the next stage, another layer of positive photoresist material isdeposited over the substrate. This layer is subjected to a multipleexposure operation involving substantially parallel radiation slanted infour directions NE, SE, SW and NW, and preferably also intermediatedirections. In this operation it will be understood that in addition tothe light shielding pattern 20, the previously formed metal regions 27,28 and 29 act as a further mask to the exposing radiation. Consequently,after the exposed resist has been developed, a resist pattern isobtained as shown by the vertical hatching in FIG. 5d comprising regions34 and 35. The region 34 corresponds to the strip 22 and metal region 27except at the region of the bridging portion 23. The region 35corresponds to the region 21 but has a reduced dimension in the E-Wdirection apart from at its ends where the underlying metal regions 28and 29 act as a mask. It is to be noted that at the area overlying thebridging portion 23 of the light shielding pattern opposing recesses areformed in the resist pattern.

A layer of negative photoresist material is deposited and subjected toexposure through the substrate with substantially parallel radiationdirected perpendicularly onto the substrate surface, as in the techniqueof Figure 1a and 1b . This resist layer is developed and then successivethin film layers of insulating material, in this example siliconnitride, and metal, for example aluminium or chromium, are deposited.Regions of the superimposed metal and insulating layers are then removedby lift-off through removal of the positive and negative resistmaterials. As a result, three portions of superimposed and co-extensiveinsulating and metal layers are left, as indicated at 36, 37 and 38 inFIG. 5e.

The complete thin film structure thus obtained is shown in plan view inFIG. 6, the materials of different parts of the structure being denotedby hatching and dotting according to the accompanying legend andcorresponding to that used in FIG. 5a-5e. The structure consists of arectangular area 40 of ITO constituting the picture element's padelectrode whose top and bottom edges are bordered with metal strips andwhose opposing sides are bordered with superimposed insulating and metallayer strips. A strip 41 comprising superimposed metal and ITO layersextends adjacent the pad electrode 40 and constitutes a portion of anaddress conductor which is common to all picture element pad electrodesof the same row. The pad electrode 40 is connected to the addressconductor 41 via a MIM device 42 which bridges these two parts.

FIG. 7 is an enlarged plan view of the region of the MIM device 42 andFIGS. 8 and 9 are cross-sectional views along the lines A--A and B--Brespectively of FIG. 7. The same hatching and dotting is used in FIGS. 7to 9 as in FIG. 6. Referring particularly to FIG. 8, the MIM device 42has a bridge portion 36 of superimposed, and co-extensive, insulatingand metal layers, here referenced 44 and 45 respectively, which bridgesthe gap between the pad electrode 40 and the address conductor 41 withrespective end parts of the bridge portion 36 overlapping edge parts ofthe ITO electrode 40 and the ITO conductor 41. At the overlap betweeneach end of the bridge portion 36 and the underlying ITO layer a MIMelement is created having an ITO-insulator-metal structure. These twoMIM elements, referenced at 46 and 47, are connected by the metal layerof the bridge portion 36. Thus each MIM device consists of two MIMelements connected in series in a lateral configuration.

For use as an active matrix substrate component in the liquid crystaldisplay device, the light shielding pattern 20 is removed and the thinfilm structure on the substrate 10 is covered by a continuous liquidcrystal orientation layer in conventional manner. The component is thenassembled together with a second substrate carrying the second set ofparallel address conductors and liquid crystal material is disposedbetween the two substrates, for example as described in U.S. Pat. No.4,683,183. The circuit configuration of the display device thus formedis shown schematically in FIG. 10 in which the second set of addressconductors and the picture elements are referenced at 49 and 50respectively. The device is driven by applying selection signals anddata signals to the row and column address conductors respectively inconventional manner. By virtue of their non-linear current/voltageproperty the MIM devices 42 demonstrate a switching characteristic.

A second embodiment of a method of fabricating a thin film structureaccording to the invention will now be described. This thin filmstructure comprises a TFT active matrix array suitable for use in a TFTtype active matrix liquid crystal display device and consists of a rowand column array of pad electrodes, crossing sets of row and columnaddress conductors and TFTs connected between each pad electrode and arespective address conductor of each set, all carried on a singletransparent substrate. The circuit configuration follows conventionalpractice with the source, drain and gate terminals of a TFT beingconnected respectively to an associated column address conductor, thepad electrode and an associated row address conductor.

This method similarly employs a single light shielding pattern appliedto the rear surface of the substrate and photolithographic processesinvolving illumination from different angles which together with liftoff procedures create required individual layer patterns on the frontsurface of the substrate.

A light shielding pattern, 54, is formed on the rear surface of thesubstrate 10 by photolithographic definition of a deposited metal layerusing a mask. A part of this light shielding pattern comprising anindividual, and typical, cell is shown in FIG. 11 in plan view lookingthrough the substrate 10. The actual pattern 54 consists of a row andcolumn array of such cells interconnected with one another, there beingone cell for each of the picture elements required in the eventualactive matrix array. Each cell pattern consists of a rectangular area 55which serves to determine a pad electrode of the required thin filmstructure, crossing strips 56 and 57 which serve to determine portionsof column and row address conductors respectively, and also part of theTFT, and an extension strip portion 58 extending from the area 54 acrossthe strip 57 which is used to determine another part of the TFT.

FIG. 12a, 12b and 12c are plan views of an area of the substratecorresponding to the cell pattern of FIG. 11 and illustrate variousstages in the fabrication process. As in the previous example, the cellpattern is represented in dashed outline and each Figure depicts theformation of an individual layer pattern of interest at a particularstage with any previously formed layer pattern being omitted for thesake of clarity.

A layer of ITO is sputtered over the substrate surface followed by asacrificial layer of a transparent material suitable for its intendedpurpose. The sacrificial layer can be of a material such as siliconnitride, silicon oxide or silicon oxynitride deposited under conditionswhich give it a predetermined, and comparatively fast, etchingcharacteristic for reasons which will become apparent. Photoresist isthen spun over the substrate and a multiple exposure photolithographicprocess is carried out in which substantially parallel radiation isdirected onto the light shielding pattern with, using the sameconvention as before N, S, E and W slanted radiation and according tothe technique of FIGS. 3a and 3b. The resist is then developed withexposed regions being removed. Exposed areas of the sacrificial andunderlying ITO layers are then etched away, after which the remainingresist is removed to leave the multi-layer pattern shown by slantedhatching in FIG. 12a. This pattern consists of rectangular area 58 ofsuperimposed and co-extensive ITO and sacrificial layers, 59 and 60respectively, generally corresponding to the area 55 of the lightshielding pattern but whose dimensions in the N/S and E/W directions aresmaller as a result of the slanting exposures employed. These exposuresresult in the regions of resist overlying the parts 56, 57 and 58 of thelight shielding pattern 54 being completely exposed to radiation, andthus the removal of the underlying regions of the sacrificial and ITOlayers.

Thereafter, a second layer of ITO followed directly by successive layersof silicon nitride and amorphous silicon (a-Si:H) are deposited over thesubstrate. The silicon nitride is deposited under conditions whichrender its etching characteristics different to that of the sacrificiallayer 60 when for example silicon nitride is also used for this layer.The amorphous silicon layer is sufficiently thin to be transparent toexposing radiation. Another layer of resist is spun over the substratesurface and the resulting structure subjected to a photolithographicprocess in which E and W slanted radiation is directed onto the rear ofthe substrate. After development of the resist the exposed regions ofthe amorphous silicon layer and the immediately underlying regions ofsilicon nitride and second layer of ITO are etched away. The remaining,unexposed, resist is then removed using acetone to leave the patternshown by slanted hatching in FIG. 12b which consists of a strip region62 and a rectangular region 63, both comprising superimposed andco-extensive layers of ITO, silicon nitride and amorphous silicon 64, 65and 66 respectively. Opposed edges of the region 63 in the E-W directionlie inwardly of the corresponding edges of the area 55 of the lightshielding pattern 54 by virtue of the exposure parameters (c.f. FIGS. 3aand 3b). It will be understood that the region 63 overlies thepreviously formed area 58 of superimposed ITO and sacrificial layers 59and 60 (not shown in Figure 12b). Because of the nature of theexposures, regions of the resist overlying the regions 56 and 58 of thelight shielding pattern are completely exposed and consequentlycorresponding regions of the layers 64, 65 and 66 are removed.

A layer of negative photoresist is then applied and subjected toexposures using N and S slanted radiation through the substrate. Theexisting, underlying, layer patterns are all transparent to theradiation. After developing the resist, layers of n+a-Si:H and a metal,such as aluminium or chromium, are deposited in succession. Regions ofthese two layers overlying the remaining regions of the resist layer arethen removed by lift-off with those resist regions.

The resulting layer pattern, comprising superimposed and co-extensivelayers 68 and 69 of n+a-Si:H and metal, is indicated by slantinghatching in FIG. 12c. This pattern consists of a rectangular region 70,determined by the area 55 of the light shielding pattern 54, whichoverlies the previously formed regions 58 and 63 and whose edges in theN-S direction lie inwardly of the corresponding edges of the area 55 andthe underlying region 63 by virtue of the exposure parameters, anextension 71 determined by the extension 58 of the light shieldingpattern which projects from the region 70 over the previously formedstrip region 62 (not shown in FIG. 12c), and a strip region 72 whichsimilarly crosses over the previously formed strip region 62. Because ofthe nature of the exposures, a region corresponding to the region 57 ofthe light shielding pattern is not defined.

To complete the thin film structure, the sacrificial layer 60 of theregion 58 is then removed by etching, taking with it the immediatelyoverlying areas of the second ITO layer 64, the silicon nitride layer 65and the a-Si layer 66 of the region 63, and the immediately overlyingareas of the n+a-Si:H layer 68 and the metal layer 69 of the region 70.Accordingly, the underlying area of the first ITO layer 59 of the region58 is then exposed. To this end, the material of the sacrificial layer60 is chosen to be selectively etchable using an etchant which does notattack the materials of any of the other layers 59, 64, 65, 66, 68 and69, and also such that it is not attacked by any etchants or solventsused in process stages subsequent to its deposition. The depositionconditions used for the layers 60 and 65 in the case for example wheresilicon nitride is used for the sacrificial layer 60, are selected suchthat the latter layer etches away at a significantly faster rate thanthe layer 65 and the layer 65 is largely unaffected by this etchingoperation although there will be some cut-back at the edges of thislayer.

It will be understood that the strip regions 62 and 72 thus defined bythe individual cell of the light shielding pattern join withcorresponding strip regions defined by adjacent cells to form continuousstrips constituting sets of row and column address conductors of therequired active matrix array. The resulting thin film structure formedon the substrate comprises a row and column array of picture elementelectrodes consisting of the exposed regions of the ITO layer 59, eachof which is connected to a configuration of layers constituting a TFTwhich in turn is connected to associated row and column addressconductors. FIG. 13 shows in enlarged plan view a part of the completedthin film structure at the region of a TFT. A cross-sectional view alongthe line C--C of FIG. 13 is shown in FIG. 14. The row address conductor,constituted by the strip region 62, or more particularly the lower ITOlayer 64, serves as a gate line for all TFTs of picture elements in thesame row. This address conductor is crossed by both the strip region 72and the extension region 71. Respective portions of these regions 72 and71 at the cross-overs together with the immediate underlying andintervening portions of the region 62 form the TFT with the respectiveportions of the regions 72 and 71 serving as source and drain contacts74 and 75, and portions of the ITO layer 64 and the SiN layer 65respectively serving as the gate 76 and gate insulator layer 77. Inoperation of the TFT, a channel region 78 is formed in the a-Si:H layer66 intermediate the source and drain regions. Upon application of a gatevoltage applied to the row address conductor 62, a voltage signalpresent on the column address conductor 72, is transferred from thesource region 74 to the drain region 75 of the TFT and then via themetal layer 69 of the extension region 71 to the ITO pad electrode 59through the layer 68.

The circuit configuration of a typical part of the thin film structurearray is illustrated in FIG. 15, in which the TFTs are referenced at 80.

In an alternative version of this embodiment, the sacrificial layer 60may be omitted and instead use made of the resist material required inthe definition of the first ITO layer 59 (and the sacrificial layer 60)to perform a similar function. In this modified method, the resist isgiven a special treatment, for instance by baking at a substantiallyhigher temperature than normal, to render it resistant to the solvent,such as acetone, used to remove subsequently deposited resist layers atlater stages in the method. The areas of specially treated resistunexposed to radiation and remaining after defining the ITO layer 59 arenot removed but are left in situ so that the subsequently depositedlayers 64, 65 and 66 overlie this area of resist, so that, in effect,the sacrificial layer 60 (FIG. 12a) is then comprised of this resistmaterial rather than a separately deposited material. At the finallift-off stage when the sacrificial layer is to be etched away a moreaggressive agent such as fumic acid is employed to remove the areas ofhardened resist.

The active matrix substrate 10 is completed by covering the structurewith a liquid crystal orientation layer and removing the light shieldingpattern 54. This substrate is assembled together with an opposingsubstrate carrying a continuous electrode and a similar orientationlayer with LC material disposed therebetween in conventional manner toproduce the display device.

The methods of both of the above-described embodiments entail a seriesof photolithographic processes requiring multiple exposures. However,unlike conventional methods requiring multiple exposures using differentmasks, these exposures are all performed using just the one lightshielding pattern, which is itself provided using just one mask. Therequired individual layer patterns are obtained simply by varying theangles of illumination in the photolithographic processes and usingstandard techniques such as temporary, sacrificial, layers and lift-offprocedures. As only one light shielding pattern is used, and becausethis is fixed in position relative to the substrate, the kind ofproblems commonly experienced in conventional methods caused byregistration difficulties and the need for accurate alignment insuccessive lithographic operations are eliminated. Moreover, since thelight shielding pattern is formed on the surface of the substrate itwill move with the substrate so that problems such as those caused bythe effects on the substrate of thermal cycling are avoided.Significantly, this enables flexible substrate materials such asplastics to be employed as well as rigid glass substrates.

Although in the above-described examples particular materials have beenmentioned for the various layers, it should be understood that othermaterials may be used as will be apparent to skilled persons. Thus, forexample, in the first embodiment insulating materials other than siliconnitride, for example, silicon oxide, silicon oxy-nitride, aluminiumoxide or tantalum pentoxide, may be used for the layer 44, the thicknessof this insulating layer being suitably selected according to thematerial used to obtain the necessary MIM action.

With regard to the described examples, it should be appreciated that inthe different photolithographic processes the angle at which theilluminating radiation meets the substrate surface when performing aslanting radiation exposure can be varied. In the example techniquesdescribed with reference to FIGS. 2a and 2b and 3a and 3b, the radiationis shown as having an angle of approximately 60 degrees with respect tothe substrate surface. This angle of inclination of the incidentradiation is important in that together with the distance between thelight shielding pattern and the layer to be defined it determines theposition of an edge of the defined layer pattern in relation to thecorresponding edge of the light shielding pattern. Therefore, for thoselayer pattern regions which are said to have dimensions less than thedimension of the corresponding region of the light shielding pattern,the extent of the dimensional differences can be dictated by appropriatechoice of the angle of the incident radiation. For radiation meeting thesubstrate surface at more acute angles, for example 30 or 40 degrees,this difference will be greater, whilst the difference will be less forgreater acute angles, for example 70 or 80 degrees. The angle ofinclination need not be the same for each exposure operation but can bechanged for individual definition processes. However, a constant angleof illumination can lead to simplification of the necessary exposureequipment.

While the invention has been described with reference particularly tothe fabrication of active matrix arrays for liquid crystal displaydevices, it should be understood that the invention is applicable alsoto fabricating thin film structures for other applications. For example,the invention can be used to produce active matrix arrays similar tothose described for use in touch sensing devices, memory devices andimage sensing devices. The invention may be used generally in thefabrication of thin film structures on transparent substrates for otherpurposes and is not limited to fabricating structures comprising arraysof elements.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art of fabricating thinfilm structures and active matrix display devices and which may be usedinstead of or in addition to features already described herein.

I claim:
 1. A method of fabricating on one surface of a transparentsubstrate a thin film structure comprising a plurality of thin filmlayers in predetermined patterns, which method comprises the steps ofproviding a light shielding pattern adjacent the opposing surface of thesubstrate and photolithographically patterning a first thin film layerdeposited over the one surface according to the light shielding patternusing radiation directed into the light shielding pattern in a firstdirection, and photolithographically patterning a second thin film layerdeposited over the one surface of the substrate using radiation directedonto the light shielding pattern in a second direction with respect tothe substrate different from said first direction used for patterningsaid first thin film layer.
 2. A method according to claim 1,characterized in that a third thin film layer is deposited over the onesurface of the substrate and is patterned photolithographically usingradiation directed onto the light shielding pattern in a directiondifferent from that used in the patterning of one of said first andsecond thin film layers.
 3. A method according to claim 2, characterizedin that the direction of radiation used in the patterning of the thirdthin film layer is different from that used in the patterning of boththe first and the second thin film layers.
 4. A method according toclaim 1, characterized in that at least one further thin film layer isdeposited successively with one of said first, second and third thinfilm layers and is photolithographically patterned simultaneously withthat layer.
 5. A method according to claim 1, characterized in that atleast one of said deposited thin film layers is patterned usingradiation which is directed onto the light shielding pattern from morethan one direction.
 6. A method according to claim 5, characterized inthat said one thin film layer is patterned using radiation in twosubstantially opposing directions which lie in a plane substantiallyperpendicular to the one surface of the substrate.
 7. A method accordingto claim 1, characterized in that a temporary thin film layer pattern isprovided over the one surface of the substrate by depositing a thin filmlayer and patterning the layer photolithographically using radiationdirected onto the light shielding pattern and in that the temporary thinfilm layer pattern is removed following the deposition of one of saidthin film layers so as to lift off immediately overlying regions of thatone thin film layer.
 8. A method according to claim 1, characterized inthat the light shielding pattern is carried on the opposing surface ofthe substrate.
 9. A method according to claim 8, characterized in thatthe light shielding pattern comprises a thin film pattern formed on theopposing surface of the substrate.
 10. A method according to claim 9,characterized in that the light shielding pattern comprises metal.
 11. Amethod according to claim 9, characterized in that the substratecomprises pliable material.
 12. A method according to any one of claim8, characterized in that following the photolithographic patterning ofthe deposited thin film layers, the light shielding pattern is removedfrom the opposing surface of the substrate.
 13. A method according toclaim 1, characterized in that the materials of the deposited thin filmlayers, the radiation directions used in the photolithographicpatterning of the deposited layers, and the light shielding pattern areselected so as to form a thin film structure comprising an array ofindividual electrodes arranged in rows and columns, a set of parallelconductors extending between adjacent rows of electrodes and a twoterminal non-linear device connecting each electrode to an associatedconductor.
 14. A method according to claim 13, characterized in that thelight shielding pattern comprises areas of radiation opaque materialarranged in rows and columns, a set of parallel strips of radiationopaque material extending between adjacent rows of said areas andbridging portions of radiation opaque material between each said areaand an associated one of set of strips.
 15. A method according to anyone of claims 1 to 12, characterized in that the materials of thedeposited thin film layers, the radiation directions used in thephotolithographic patterning of the deposited layers, and the lightshielding pattern are selected so as to form a thin film structurecomprising an array of individual electrodes arranged in rows andcolumns, first and second sets of parallel conductors extending betweenrespectively adjacent rows and adjacent columns of electrodes, and athin film transistor between each electrode and an associated conductorof each set.
 16. A method according to claim 15, characterized in thatthe light shielding pattern comprises areas of radiation opaque materialarranged in rows and columns, first and second sets of parallel stripsof radiation opaque material which cross one another and extend betweenadjacent rows and columns of said areas, and portions of radiationopaque material which extend between each said area and an associatedone of the first set of strips.